Introduction: The aim of this paper is to present a new method for the estimation of free radicals (FR) in urine. First of all it turned out that a relatively very difficult part of our work was to find an appropriate group of patients, with positive values of FR concentrations in urine. The most serious of nephrological diseases cause only a significant decrease of the renal antioxidant capacity which results in presence of FR in urine. Material and methods: Urine samples, separated from 24-hour diuresis and serum samples from the same of patients (total No: 28 – G1) were stored (–18 °C) and assembled; then all the samples were defrosted at the same time and FR in both biological fluids were immediately estimated by the original method, based on the chlorophylline reaction. The method is routinely used for FR determination in serum. As control group served patients of the nephrologic clinic suffering from various types of kidney disorders (G0), without damage of the renal antioxidant capacity. Results: The following FR values were found: Serum: G1: 2.51 ± 0.21 mgFe 2+ , G0: 2.28 ± 0.18 mgFe 2+ Urine: G1: 2.31 ± 0.56 mgFe 2+ , G0: no FR were found in urine of those patients. No statistically important correlation between serum and urine samples was found. The Wilcoxon test was used for statistical evaluation of results. Discussion: Based on the obtained results we can assume that the submitted method can be used for the verification of the most serious damage of renal function. An additional possibility how to exploit the FR values in urine is probably the area of therapy: if the amount of FR urinary excretion falls to zero it can be assumed, that the selected form of treatment seems successful. Conclusion: Our method can serve as a marker of renal antioxidant capacity in diagno sis, as well as in therapy of the most serious types of renal diseases.

        Key words: free radicals, chlorophylline, urine, kidney.